1. Field of the Invention
The present invention relates to fire extinguishing systems installed on a flammable chemical bath within a room, such as a clean room, a laboratory or a chemical treatment room for components manufactured, where flammable chemicals are handled and yet is inaccessible for fire engines or the like.
2. Description of the Prior Art
Facilities for manufacturing electronic components, especially those electronic components requiring a precise working such as semiconductor devices, are arranged in a closed room having a high air cleanliness such as a clean room. These facilities include an exposure system, a diffusion facility or a chemical treatment bath. Of these facilities one which tends to catch fire is the chemical treatment bath. In particular, a chemical bath for cleaning which is one type of the chemical treatment bath is liable to catch fire. This is because the bath is storing a flammable chemical such as isopropyl alcohol.
Generally, a chemical treatment facility of the above-mentioned type is constituted of a large number of baths respectively containing various kinds of chemicals that are arranged in one direction, and a carrier which runs over these baths by having on board the components to be treated. Accordingly, if fire occurs in one bath, fire spreads to other baths by leaping flames, and there is a hazard of eventually reducing the building to ashes.
As a fire extinguishing facility for baths of flammable chemicals of the above-mentioned kind, system that jets out an incombustible gas which shuts off the inflow of the air has been employed in order to suppress the contamination or damage that occurs at the time of fire extinguishing to a possible minimum level.
A fire extinguishing facility of this kind is constituted of a fire detector which is arranged in the neighborhood of the chemical baths which are the objects of extinction and detects the occurrence of a fire, a valve which opens its valve seat in response to the detection signal of the fire detector, a carbon dioxide container connected to one opening of the valve through a pipeline for supplying carbon dioxide as an extinguishing gas, and a carbon dioxide injection nozzle which is connected to the other opening of the valve and is arranged in the region where the chemical baths are installed, as is disclosed, for example, in the catalog entitled "Series 27100, 2800 Detect-a-Fire.RTM. Vertical Units" prepared by Fenwal, Inc. (400 Main Street, Ashland, Mass., U.S.A.).
When the fire detector detects the temperature rise, infrared rays or ultraviolet rays due to the occurrence of a fire, the detector issues a signal, the valve is opened in response to the signal, carbon dioxide is supplied to the injection nozzle from the carbon dioxide container, and carbon dioxide is discharged from the injection nozzle toward the chemical bath installation region to obstruct the supply of the air to the installation region in order to lead to extinction.
Now, carbon dioxide used for the fire extinguishing system has the power of obstructing the inflow of the air to the region of fire occurrence, but it lacks the fire extinguishing action. Accordingly, considerable quantity of carbon dioxide is needed to obtain the expected effect. Moreover, when a large quantity of carbon dioxide is jetted out into a closed room such as a clean room, the entire interior of the room finds itself in an oxygen deficient condition.
For this reason, the recent trend is to use a halogenated hydrocarbon (trade name "Halon" made by Du Pont, Corp.) in place of carbon dioxide. The characteristics of Halon are as follows.
1. That it has a strong chemically negative catalytic effect, that is, it has a strong action to stop the combustion chain reaction, and it has a strong combustion suppressing action (the quantity of gas needed for extinction is approximately one third of that of carbon dioxide).
2. That it is a poor electrical conductor.
3. That it does not react with metals, so that there is hardly any contamination of metals accompanying the gas discharge at extinction.
4. That it is harmless to man and beast.
5. That it is extremely stable chemically so that the periodic exchange which is ordinarily required for other extinguishing reagents is not necessary.
An example of fire extinguishing systems constructed by using Halon that possesses the above-mentioned characteristics in place of carbon dioxide is an apparatus which is put in the market by Nomi Disaster Prevention Industrial Co. under the name of "Halon 1301 type Fire Extinguishing System". This system sharply reduces the required quantity of the extinguishing reagent compared with the system employing carbon dioxide, by making an advantageous use of the aforementioned characteristics of Halon gas. Moreover, utilizing the low contamination property listed as the third item of the characteristics of Halon, this fire extinguishing system has become to be in widespread use not only for the cleaning tanks for electronic components but also for the treatment baths where etching and surface working treatment take place.
However, Halon is an expensive material so that the cost runs high even if the required quantity is little. Furthermore, when it is thermally decomposed at high temperatures, it generates fluorides because it is a halogenated hydrocarbon, and the fluorides thus generated spoils the earth's environments by destroying the ozone layer above the earth. Because of this, it was decided in the Working Committee meeting for Protocol Amendment held at Montreal in November, 1989 that the use of the substance be wholly abolished by the year 2000.